Hearth for a heat treating furnace



Jan. 14, 1969 K. H. sEELAND-r ETAL 3,421,747V

HEARTH A HEAT TREATING FURNACE Filed Jan. 18, 19e? sheet of 2 Jan. 14, 1969 K, H, SELANDT ETAL HEARTH FOR A HEAT TREATING FURNACE of 2y Sheet- Filed Jan. 18, 1967 mvsww-QRJ @dar-j QJ. CeeQnc/C United States Patent O FURNACE Rockford, Ill., Philadelphia,

ABSTRACT F THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to a furnace for heat treating workpieces and, more particularly, to a furnace of the type in which a hearth disposed within the furnace chamber supports the workpieces While the latter are being treated under high temperatures and often under high orders of vacuum.

SUMMARY OF THE INVENTION The present invention aims to provide in a furnace of the above character a new and improved hearth which, as compared with previous constructions, is more rugged and trouble-free in service use, is less likely to break due to thermal shock or to distort under mechanical loads and, at the same time, is more easily and economically repaired in case of damage caused by improper handling of the workpieces. To achieve these ends, the hearth is formed as a series of workpiece-supporting bars made of material having -properties of high thermal conductivity but low thermal expansion thereby to reduce exp-ansion and contraction of the bars as the furnace is heated and cooled and thus minimize breakage resulting from thermal shock. In addition, the bars are composed of a material which possesses low creep characteristics under conditions of high temperature in order to prevent the bars from distorting when mechanically loaded with heavy workpieces.

To avoid diifusional bonding between the material of the hearth bars and the material of the workpieces, nonmetallic caps are placed on the upper sides of the bars to form protective shields over the bars and prevent the bars and the workpieces from combining molecularly in the heated and evacuated environment of the furnace chamber. The caps, as well as the' bars, are removable and replaceable individually to facilitate rapid and economical repair should breakage of these parts occur as a result of rough handling of the workpieces by the furnace operator.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a transverse sectional view of a furnace equipped with one form of a hearth embodying the novel features of the present invention. f

FIG. 2 is an enlarged fragmentary cross-section taken substantially along the line 2-2 of FIG. 1.

FIG. 3 is an enlarged side elevation of parts shown in FIG. 1.

PIG. 4 is a fragmentary cross-section taken substantially along the line 4-4 of FIG. 2.

Patented Jan. 14, 1969 FIG. 5 is a perspective view of a part shown in FIG. 3.

FIG. 6 is a fragmentary perspective view of a furnace equipped with a modified hearth.

FIG. 7 is a fragmentary side elevation of the hearth shown in FIG. 6 with parts being broken away and shown in section.

FIG. 8 is a fragmentary cross-section taken substantially along the line 8-8 of FIG. 7.

DETAILED DESCRIPTION As shown in the drawings for purposes of illustration, the invention is embodied in a vacuum heat treating furnace 10 of the type having a heating chamber 11 defined within a walled enclosure including a top wall 13, a bottom Wall 14 and laterally spaced side Walls 15, the walls herein being formed by an external shell 16 and an internal liner 17 separated from each other by a layer of refractory material 19. During treatment, metal workpieces 20 placed on a supporting hearth 21 are heated to high temperatures by electric radiant heaters (not shown) extending horizontally across the chamber. The workpieces may be loaded onto the hearth by a conveyor or a fork lift and may be supported either directly on the hearth or in metal trays or baskets. To prevent oxidation of the workpieces during heating, the chamber 11 usually is evacuated prior to heating by vacuum pumping apparatus (not shown).

We have `discovered that, by constructing the hearth 21 in a novel manner as a series of individual bars 23 made of a material having low creep characteristics and having properties of low thermal expansion and high thermal conductivity, the hearth is less likely to distort under heavy mechanical loading and is less likely to break due to thermal shock occurring as the furnace 10 is heated and cooled. Moreover, non-metallic caps 24 placed on the upper or supporting surfaces of the bars insulate the latter chemicallly from the metal workpieces 20 to prevent dilfusional bonding between the material of the bars and the material of the workpieces under conditions of high temperature and vacuum.

In the present instance, each hearth bar 23 (only one 0f which is shown in FIG. l) is rectangular in cross-section and extends transversely betwen the furnace side walls 15 above the level of the bottom wall 14. Preferably, the bars are made of graphite which, as compared with most metals or with most ceramics conventionally used in furnace hearths, is of high thermal conductivity but of low thermal expansion. As a result, the amount of expansion and contraction occurring in the graphite material as the chamber 11 is heated and cooled is relatively low and thus the bars are less susceptible to breakage caused by thermal shock. In addition, graphite possesses low creep characteristics, and increases in load bearing capacity in response to temperature increases so that the tendency of the hearth bars to become permanently distorted when heavily loaded with workpieces under high temperature conditions is significantly reduced. Graphite is suitable for use under high orders of vacuum and, unlike many other hearth materials, does not vaporize excessively in the evacuated and heated chamber. Certain high-temperature metals, such as tungsten and molybdenum, possess similar properties and may be used to similar advantage as the constructional material of the bars, lbut graphite is preferred since it is considerably less expensive than the metals and thus results in a more economical construction.

As shown most clearly in FIGS. l and 2, the insulator caps 24 herein are in the form of tubular rollers which are made of ceramic or other non-metallic and heat-resistant material and which are seated loosely in an upwardly opening longitudinal groove 25 for-med in the upper surface of each hearth bar 23. The ceramic rollers disposed between the graphite bars and the metal workpieces 20 prevent the graphite from combining molecularly with the workpieces. Accordingly, the characteristics of the metal of the workpieces are not changed by diffusional bonding or growing together of the graphite and the workpieces that otherwise might occur at the contact points if the workpieces were supported directly on the graphite bars under conditions of high temperature and vacuum.

Preferably, the rollers 24 are formed as a row of short segments, herein six such segments, disposed end-to-end in the groove 25. Being segmented and of short length, the rollers not only are less likely to break due to thermal shock but also each roller is replaceable independently of the other rollers should one or more of the segments being broken as a result of the operator dropping the workpieces 20 on the hearth or otherwise loading the workpieces in a rough manner.

In the embodiment of the hearth 21 shown in FIG. l, each bar 23 is supported at its ends -by the furnace side walls and intermediate its ends by a pair of posts 26 upstanding from the bottom wall 14. Each end of the bar extends outwardly through a hole formed in the adjacent side wall and is enclosed by a rectangular block 27 (FIGS. 3 and 5) of rigid ceramic insulating material housed within a casing 29 detachably fastened to the exterior shell 16 of the side wall by screws 30 extending through a ange 31 integral with the upper end of the casing. A layer 33 (FIG. 3) of ceramic wool is wrapped around the portion of the bar disposed within the hole and this, together with the insulating block 27, restricts the escape of heat from the chamber I11 to the outside of the furnace along the highly conductive hearth bar.

The supporting posts 26 preferably are made of the same conductive material as the hearth bars 23 and are telescoped at their lower ends into steel locating rings 34 (FIGS. 2 and 4) welded to the inner side of the external shell 16 of the bottom wall 14. A sleeve 35 of ceramic wool lining each ring and a wafer 36 of foamed zirconia disposed between the external shell and the lower end of each post thermally insulate the latter from the shell. As shown in FIG. 2, each post projects upwardly through the internal liner 17 of the bottom wall and is formed at its upper end with a short cylindrical knob 37. The latter fits into and mates with a correspondingly shaped recess formed in the lower side of the bar to prevent sidewise and longitudinal movement of the bar relative to the post. Should a bar be broken because of rough loading of the workpieces 20, it may be removed independently of the other bars for purposes of replacement simply by detaching one of the casings 29 from the side wall 15, lifting the bar upwardly off of the knobs 37 on the posts, and sliding the bar endwise through the opening in the side wall.

A modified hearth is shown in FIGS. 6 to 8 in which similar parts are indicated by the same but primed reference numerals. In this instance, the hearth bars 23 are disposed entirely within the furnace chamber and are supported solely by upright posts 26 formed with knobs 37. It is apparent that a damaged bar may be removed from the furnace independently of the other bars simply by lifting the bar off of the knobs on the posts. At its lower end, each post is formed with a cylindrical knob 40 (FIG. 7) which ts into a similarly shaped recess formed in the upper side of a wafer 36 of foamed Zirconia. Preferably, a ceramic tube 41 is telescoped over the portion of the post disposed between the bar and the wafer to insulate the post electrically and thus shield the post from the electrical heating elements in the furnace chamber.

As shown most clear-ly in FIGS. 7 and 8, each wafer 36 is bonded to a fiat disk 43 which rests on the upper ends of a pair of adjusting screws 44 threaded through the external shell 16' of the bottom wall 14 of the furnace. Should the bottom wall warp after extensive use,

the posts 26' may be raised and lowered from time to time yby adjustment of the screws to position the worksupporting surfaces of all of the bars 23 in a common plane and also to maintain the work-supporting surface of each bar in a horizontal plane.

The caps 24 for preventing diffusional bonding between the workpieces and the hearth bars 23' are in the form of inverted U-shaped ceramic blocks which are telescoped downwardly over the bars. As in the first embodiment, a number of blocks 24' are disposed end-toend on each bar and are of short length to increase their resistance to breakage by thermal shock and to facilitate economical replacement. A layer (FIG. 8) of heatresistant fibrous material such as graphite felt is sandwiched between the blocks and the bars and cushions mechanical shock resulting from the loading of the workpieces.

From the foregoing, it will be seen that a new and improved hearth constructed in accordance with the present invention has an extremely long service life since the hearth bars are highly resistant to breakage and distortion caused by thermal shock and mechanical loading. The ceramic caps prevent difiusional bonding between the bars and the workpieces and are constructed so as to be replaceable in a rapid and economical manner.

We claim as our invention:

l. In a heat treating furnace, the combination of, an enclosure having a bottom wall and defining a work chamber, a support in said chamber for workpieces to be treated, said support comprising a series of elongated hearth bars spaced upwardly from said bottom wall and made of material having high thermal conductivity and low thermal expansion and capable of withstanding heavy mechanical loading under high temperature conditions, a plurality of posts upstanding from said bottom wall and supporting said hearth bars intermediate their ends, and a row of elongated caps disposed end-to-end and resting loosely on the upper surface of each bar to form the work-supporting surface of the bar with each cap of the row being removable individually from the bar, said caps being made of non-metallic and heat resistant material to prevent diffusional bonding between the material of the bars and the material of the workpieces under conditions of high temperature.

2. The combination defined in claim 1 in which said hearth bars are formed with longitudinal grooves in said upper surfaces, and said caps rest loosely in said grooves.

3. The combination defined in claim 2 in which said caps are cylindrical rollers seated loosely in said grooves.

4. The combination defined in claim 3 in which said rollers are tubular and are made of ceramic material.

5. The combination defined in claim 1 in which said caps are inverted U-shaped blocks telescoped downwardly over said bars.

6. The combination defined in claim 5 further including a cushion of heat-resistant fibrous material sandwiched between said bars and said blocks.

7. The combination defined in claim 1 in which saide hearth bars are composed of graphite.

8. The combination defined in claim 7 in which said posts are composed of graphite.

9. The combination defined in claim 8 further including sleeves composed of insulating material .telescoped over and surrounding said posts.

10. In a vacuum heat treating furnace, the combination of, a walled enclosure defining a work chamber, a support in said chamber for workpieces to be treated, said support comprising a series of elongated hearth bars composed of graphite, and caps of ceramic material disposed on the upper surfaces of said bars to form the work-supporting surfaces of the bars and thereby prevent diffusional bonding between the graphite and the workpieces under conditions of high temperature and high orders of vacuum.

11. The combination defined in claim 10 in which each cap comprises a row of ceramic rollers disposed end-toend and resting loosely on the upper surface of said bar with each roller of the row being removable individually v from the bar.

12. The combination dened in claim 11 further including a plurality of posts composed of graphite upstanding from the bottom wall of said enclosure and supporting said hearth bars intermediate their ends.

13. In a vacuum hea-t treating furnace, the combination of, a walled enclosure delining a work chamber, a support in said chamber for workpieces to be treated, said support comprising a series of elongated hearth bars made of material having high thermal conductivity and low thermal expansion and capable of withstanding heavy mechanical loading under high temperature conditions, and a cap of non-metallic and heat-resistant material disposed on the upper side of each bar to form the work-supporting surface of the bar and thereby prevent diffusional bonding between the material of the bar and the material of the workpieces under conditions of high temperature and high orders of vacuum.

14. In a heat treating furnace, the combination of, an enclosure having a bottom wall and deining a work chamber, a support in said chamber for workpieces to be treated, said support comprising an elongated hearth bar spaced upwardly from said bottom wall, a pair of laterally spaced posts upstanding from said bottom wall and connected at their upper ends to the lower side of said bar intermediate the ends of the bar, and vertically adjustable means disposed between said posts and said bottom wall whereby said posts may be raised and lowered independently to adjust the elevation of said bar.

15. The combination defined in claim 14 in which said support includes a series of elongated hearth bars, each of said bars resting loosely on its respective posts and being removable from said chamber independently of the other bars.

16. The combination dened in claim 1 in which said hearth bars are made of a material selected from the group consisting of graphite, tungsten and molybdenum.

References Cited UNITED STATES PATENTS 799,744 9/1905 Mackay 263-47 852,000 4/ 1907 Sklovsky 263-2 1,037,665 9/1912 Rockwell 263-2 1,649,926 ll/ 1927 Ruckstahl et al 263-2 X 2,105,838 1/ 1938 McCormick. 2,492,220 12/ 1949 Hasselhorn 263-2 2,837,326 6/1958 Muth 263-40 JOHN I. CAMBY, Primary Examiner.

U.S. Cl. X.R. 

